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Get It Plated Right

This fact sheet series is produced
by the Minnesota Association of Metal Finishers &
Minnesota Technical Assistance Program for metal fabricators
and their platers.

Selecting Materials for Plated Parts

Fact Sheet #3

The type of metals used to construct
a part can affect how easily it is cleaned and the part's
physical condition after a finishing process. If dissimilar
metals are used without regard to how the combination
reacts to finishing chemicals, severe problems can result.

Dissimilar metals are typically brought
together through joining processes. Problems with welded,
brazed or soldered assemblies are most common, but bolted
and press-fit assemblies must also be dealt with carefully.

Mixed Steel and Stainless Steel Assembly
Triples the Plating Cost

A manufacturer of food packaging machinery
attempted to lower costs on one of their larger weldments,
by welding stainless steel plates to a cold-rolled steel
tube. The assembly was sent out for electroless nickel
plating to protect the steel tube from corrosion during
use. By oversight the plater was not told the plates
were stainless steel.

When the plated parts came back, the
manufacturer found small blisters forming on the stainless
steel. Plating in these areas easily scraped off. But,
the plating on the steel tube was acceptable. The manufacturer
called their plater and learned electroless nickel--as
well as most other types of plating--does not bond well
to stainless steel.

To salvage the parts, the plating
was stripped and the parts received a nickel strike
followed by replating with electroless nickel. The
total cost was three times the original quote. If
the plater had been able to prepare for the dissimilar
metals, a nickel strike step could have been inserted
into the process, giving adequate bonding to the entire
part, for about a 15% increase in plating costs.

Problem Sources and Solutions

The manufacturer had problems
when their plating shop had insufficient knowledge about
the part they were finishing. The problem could have
been avoided through better communication when the job
was arranged. Alternatively, this part could have been
made entirely out of cold-rolled steel and plated, or
entirely made of stainless steel and left without plating,
avoiding issues of adhesion entirely. The best choice
depends on overall economics and how the part will be
used.

The selection of the base metal of
a part and the type of plating applied have a strong
influence on the plating bond strength. Given good preparation
and processing the bond strength for substrates and
finishes vary according to the following table.

In addition, the bond strength of
plate on a hardened alloy will be lower than for the
same plate on the same unhardened alloy. The problem
with chrome alloys, and to a lesser extent nickel and
leaded steel alloys, is that these metals oxidize in
the presence of air. The same thing that gives chrome
alloys corrosion resistance, makes the surface difficult
to activate and creates weak bonds with the plated metal.
Weak bond strengths can be partially compensated for
by increasing the thickness of plating.

Welded stainless steel assemblies
are troublesome for two reasons: 1) plated coatings
do not bond well because of its chromium content; and
2) welding causes further oxidation and changes the
local metal composition slightly.

Stainless steels and some other difficult
metals can be plated successfully if surfaces are prepared
with a thin, nickel or copper strike. However, plating
bond strength will be roughly half as good with carbon
steel. Also, strikes are off-line, specialty plating
baths. They are always an extra process step, used with
another plating bath which produces a more corrosion
resistant, wear resistant, or aesthetic finish. Another
disadvantage is the nickel strike coating does not penetrate
into recesses, holes and tubing nearly as well as electroless
plating. If plating quality is important on these hard
to reach surfaces, the part design or metal selection
needs to change. When strikes are required, plating
costs increase.

Dissimilar metals in an assembly can
also cause problems, especially when that information
is not communicated to the plater. Frequently one of
the metals will etch or corrode in the strong plating
chemicals, which would not occur in the absence of the
other metal. The dissimilar metals plus the plating
chemical set up an electrochemical corrosion cell. This
can result in: stains; a roughened texture in the corroded
areas; an unintended metal coating on other metal surfaces,
affecting either the part appearance or the plating
quality; or in extreme cases, substantial loss of material
in the assembly.

Procedural Solutions

Change welding rods when
jobs change, to match the base metal of the new job.
If an operator continues to use stainless steel wire
or rod for carbon steel parts, in general the weld
will perform well structurally, but plating on the
welds will flake. Other metal mismatches will generally
perform better than combinations of steel and stainless
steel, but adhesion will be compromised.

Use only stainless steel brushes
and tools to remove slag and scale from welds
on stainless surfaces, to prevent imbedding impurities
which can create defects over time.

Avoid brazing with copper-zinc
filler on ferrous parts that will be plated. Use
silver solder or join components after plating.
Copper dissolves in acid baths and leaves a thin immersion
copper plate on steel surfaces which interferes with
adhesion of the intended plate.

Avoid tin-lead soldering for parts that will be plated. Use silver solder
instead. There are no good conventional procedures
to plate over lead. In addition, lead contaminants
in plating baths add significantly to platers' expense
and difficulty of waste management.

Mask the stainless steel or
difficult to plate components and plate only the steel.
Realize this process can still cause slight flaking
at the weld joints if the masking is not perfect.

Design Solutions

Use cold-rolled steel whenever possible
in assemblies to be plated. Hot-rolled steels have
carbon scales on their surfaces which interfere with
plating bonds.

Clean surfaces before welding if hot-rolled steel must be used. Welding hot-rolled
steel increases the difficulty of removing scale.
Clean surfaces by abrasive blasting or immersion in
a strong acid pickle.

Avoid mixing metal parts in an assembly that will be plated or chemically finished.
The presence of more than one metal can create a corrosion
cell within a part which has been known to completely
destroy components by treating them as sacrificial
anodes. When significant destruction does not occur
staining is quite common.

Assemble parts after plating if the use of dissimilar metal components is unavoidable.
Join with screws, bolts, rivets or press-fit, as appropriate.

Avoid welding stainless steel and steel components together. To prevent flaking
or peeling of the plating, coat the whole assembly
with a nickel strike before plating if the use of
both is unavoidable.

Consider polishing as an
alternative to electroplating stainless steel assemblies.

Other Examples of Problem Combinations

Mixed aluminum alloys being anodized are susceptible to sacrificial corrosion.
Alloys 6061 and 2024 (the anode) are an extreme case
where the 2024 part is likely to be destroyed during
anodizing.

Steel inserts, and most
other types of metal inserts, in aluminum parts being
anodized can be severely etched. The heat generated
by the acid attacking the insert can burn the surrounding
aluminum. Titanium is the only metal which is safe
in the presence of aluminum during anodizing.

Stainless pemnuts, pop
rivets and similar hardware attached to steel
parts undergoing zinc plating are a common problem
in which plating will not adhere to the stainless
steel without a nickel strike coating.